router: Rewrite the ingress MTU to one configured for the interface

There are edge cases where routers forward ICMPv6 RA which contain an MTU which
is too large/small for the receiving client. Override the ingress MTU with the
value configured on the interface.

Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/296
Signed-off-by: Álvaro Fernández Rojas <[email protected]>

diff --git a/src/router.c b/src/router.c
index 82d3187d1cb65960674ff3596803484ce4167ade..9b482d2d9dcab45774728644062bc227a642394f 100644 (file)
--- a/src/router.c
+++ b/src/router.c
@@ -1089,19 +1089,34 @@ static void forward_router_advertisement(const struct interface *iface, uint8_t
        uint8_t *mac_ptr = NULL;
        struct in6_addr *dns_ptr = NULL;
        size_t dns_count = 0;
+       // MTU option
+       struct nd_opt_mtu *mtu_opt = NULL;
+       uint32_t ingress_mtu_val = 0;
 
+       /* Parse existing options */
        icmpv6_for_each_option(opt, &adv[1], end) {
-               if (opt->type == ND_OPT_SOURCE_LINKADDR) {
-                       /* Store address of source MAC-address */
+               switch (opt->type) {
+               case ND_OPT_SOURCE_LINKADDR:
                        mac_ptr = opt->data;
-               } else if (opt->type == ND_OPT_RECURSIVE_DNS && opt->len > 1) {
-                       /* Check if we have to rewrite DNS */
-                       dns_ptr = (struct in6_addr*)&opt->data[6];
-                       dns_count = (opt->len - 1) / 2;
+                       break;
+
+               case ND_OPT_RECURSIVE_DNS:
+                       if (opt->len > 1) {
+                               dns_ptr = (struct in6_addr *)&opt->data[6];
+                               dns_count = (opt->len - 1) / 2;
+                       }
+                       break;
+
+               case ND_OPT_MTU:
+                       if (opt->len == 1 && (uint8_t *)opt + sizeof(struct nd_opt_mtu) <= end) {
+                               mtu_opt = (struct nd_opt_mtu *)opt;
+                               ingress_mtu_val = ntohl(mtu_opt->nd_opt_mtu_mtu);
+                       }
+                       break;
                }
        }
 
-       notice("Got a RA on %s", iface->name);
+       info("Got a RA on %s", iface->name);
 
        /* Indicate a proxy, however we don't follow the rest of RFC 4389 yet */
        adv->nd_ra_flags_reserved |= ND_RA_FLAG_PROXY;
@@ -1140,8 +1155,16 @@ static void forward_router_advertisement(const struct interface *iface, uint8_t
                        }
                }
 
-               notice("Forward a RA on %s", c->name);
+               /* Rewrite MTU option if local RA MTU is configured */
+               if (c->ra_mtu && mtu_opt) {
+                       if (ingress_mtu_val != c->ra_mtu) {
+                               debug("Rewriting RA MTU from %u to %u on %s",
+                                      ingress_mtu_val, c->ra_mtu, c->name);
+                               mtu_opt->nd_opt_mtu_mtu = htonl(c->ra_mtu);
+                       }
+               }
 
+               info("Forward a RA on %s", c->name);
                odhcpd_send(c->router_event.uloop.fd, &all_nodes, &iov, 1, c);
        }
 }